Are we getting overcharged for using a sponsor?

It is unclear from your information as to what the attorney is charging for. Most employment-based immigration (green card) cases require three steps, the first of which is "labor certification". The fee for this varies significantly depending on the attorney and your location, but typically ranges from $3500 to $7500 plus expenses. The next two steps are significantly less costly. You can always call other attorneys in your area, consult with them and compare fees. You should be comfortable with your attorney regardless of the fee structure.